Back in L.A., Theron, 38, and Penn, 53, were spotted making a Starbucks run together on Saturday. While Theron picked up coffee, Penn waited for her in the car.

"They both were smiling," a witness tells PEOPLE, adding that Penn seemed "very happy and relaxed."

After driving to Theron's house, the Oscar winners emerged the next day for a trip to the movies, catching The Wolf of Wall Street on Sunday night at the ArcLight Hollywood.

Reps haven't commented on the relationship, but there's been chemistry between the longtime friends before. According to a source, the duo were "touchy-feely" at Seth MacFarlane's Halloween party last October. Then, over New Year's, Theron joined Penn at his beachfront home on Oahu, and they flew back to L.A. together.